High-hardenability, medium-carbon, low-alloy round steel for fasteners and the manufacturing method thereof

ABSTRACT

The present invention relates to a high-hardenability, medium-carbon, low-alloy round steel for fasteners, the chemical constituents by mass percentage are as follows: C: 0.36˜0.44%, Si: 0.15˜0.40%, Mn: 0.80˜1.00%, Cr: 1.00˜1.15%, Mo: 0.05˜0.25%, Ni: 0.05˜0.25%, Cu: 0.05˜0.25%, Al: 0.015˜0.050%, B: 0.0010˜0.0050%, Ti: 0.020˜0.050%, the balance is Fe; the maximum diameter of the round steel is 65 mm. The manufacturing process are as follows: the raw materials are processed, in sequence, by converter smelting, LF refining, RH/VD degassing to obtain molten steel, feeding Ti wires and ferroboron, continuous casting, rolling into the bar, obtaining the quenched and tempered round steel after quenching and tempering treatment; the quenched and tempered round steel is able to be directly used in processing fasteners which meet ISO 898-1 standard for grade 10.9, such as bolts and the like.

TECHNICAL FIELD

The present invention relates to a field of manufacturing special roundsteel, and specially relates to a high-hardenability, medium-carbon,low-alloy round steel, which satisfies the mechanical performancerequirements of Ø65 mm fasteners after quenching and temperingtreatment, and the manufacturing method thereof.

BACKGROUND ART

At present market, the fasteners are manufactured mostly according tothe requirements of standard ISO898-1: Mechanical properties offasteners—Part 1: Bolts, screws and studs. The table 3 of this standardputs forward very strict requirements of mechanical and physicalperformance for fasteners, wherein the difficulties are: {circle around(1)} after quenching, the martensite structure in core of the steelshould be 90% or more; {circle around (2)} on the basis of satisfyingthe strength and the hardness in each strength grades, the difference ofHV 0.3 hardness between surface and core should be less than 30 HV;{circle around (3)} taking a whole tensile sample whose tensioned parthas a diameter of 75% to the diameter of fastener for testing themechanical performance of strength, elongation, reduction of fractureand etc.; {circle around (4)} on the basis of satisfying a high-gradestrength and plastic properties, the impact energy Akv 2 measured at−20° C. is equal to or more than 27 J. It requires that the steel forfasteners has good hardenability, and sufficient depth of quenched zonecan be obtained in quenching, so as to ensure martensite percentage,thereby to ensure the uniformity of structure and hardness of thefinished product to match the strength and toughness. Bigger is the sizeof the fastener, requirements of hardenability of the steel is higherand the difficulty of manufacturing is greater. At present, the steelssuch as 32CrB4 or 42CrMo etc. are used to manufacture fasteners ofdiameter less than 45 mm, the steels such as 40CrNiMo, 4140MOD etc. withhigher contents of noble metals like Mo and Ni are used to manufacturefasteners of diameter equal to or more than 45 mm. Otherwise, the corequenched structure, the hardness distribution in transversalcross-section, and the strength and toughness would not satisfy thematching requirements.

DISCLOSURE OF INVENTION

Aiming at above technical requirements, the technical problem to besolved in the present invention is to provide a round steel of CrB classwith medium carbon and low alloy, to replace the steels such as40CrNiMo, 34CrNiMo6 etc. having high content of Mo and high content ofNi. It can manufacture fasteners with size up to 65 mm and withmechanical performance satisfying standard of ISO 898-1, and achieve thepurpose of reducing the cost.

The solution in the present invention is as follows: ahigh-hardenability, medium-carbon, low-alloy round steel for fasteners,wherein its chemical constituents by mass percentage are as follows: C:0.36˜0.44%, Si: 0.15˜0.40%, Mn: 0.80˜1.00%, Cr: 1.00˜1.15%, Mo:0.05˜0.25%, Ni: 0.05˜0.25%, Cu: 0.05˜0.25%, Al: 0.015˜0.050%, B:0.0010˜0.0050%, Ti: 0.020˜0.050%, the balance is Fe and unavoidableimpurity elements. The maximum diameter of the round steel is 65 mm,after quenching and tempering treatment, its mechanical structure andperformance meet the requirements of ISO 898-1 standard or otherequivalent standards for grade 10.9 fasteners, specific requirements areas follows: quenched martensite content is

90% after quenching and tempering treatment, when a tensile sample istested, the diameter of a tensioned part of the sample is 75% of nominalsize, yield strength of the round steel Rp0.2 is

940 MPa, tensile strength Rm is 1040˜1140 MPa, elongation is z,28 9% ,reduction of area at fracture is z,28 48% Charpy impact energy measuredat −20° C. is z,28 27 J when a Akv2 impact sample is taken at ½ radius,HV0.3 hardness of the whole cross-section of round steel is 320˜380 HV,difference of the hardness of the whole cross-section is within 30 HV.

The chemical constituents of round steel in the present invention aredetermined by this way:

C is used for improving hardenability, strength and hardness, but itdecreases plasticity and toughness, and increases ductile-brittletransition temperature. Medium carbon content is applied in the presentinvention. The C content is controlled to be 0.36˜0.44%.

Si is a deoxidizing element in steel, and is used for increasing thestrength by solid solution strengthening. When Si content is lower than0.1%, the deoxidation effect is poor, when Si content is too high, thetoughness is reduced. The Si content is controlled to be 0.15˜0.40%.

Mn is an element for improving hardenability of steel, and has effect ofsolid solution strengthening to increase the strength. But excessive Mncontent is easy to reduce the plasticity and toughness of steel. Inorder to match the strength, plasticity and toughness, the Mn content iscontrolled to be 0.80˜1.00%

Cr and Mo are used for improving hardenability and improving thestrength and toughness of material, Mo also has effects of reducingductile-brittle transition temperature, inhibiting temperingbrittleness, improving precipitation strengthening of carbon and niobiumnitride, inhibiting massive ferrite, and hindering P segregation.However, Mo is a noble metal element, excessive addition increases themanufacture cost. So the Cr content is controlled to be 1.00˜1.15%, Mocontent is controlled to be 0.05˜0.25%.

Ni is an element for improving hardenability of steel and obviouslyimproving the toughness under low temperature, it has good influence onimpact toughness and ductile-brittle transition temperature. Inaddition, Ni is also a noble metal element, excessive addition increasesthe manufacture cost. Considering all the above, the Ni content iscontrolled to be 0.05˜0.25%, helping to achieve the best costperformance.

Cu can improve hardenability of steel. However, excessive addition of Cuis easy to cause copper brittleness, and worsens the surface property.The Cu content of the present invention is 0.05˜0.25%.

B is an element which most obviously improves the hardenability ofsteel, and plays an important role in the present invention in replacingnoble metals Mo and Ni, thereby to reduce cost. The B content of thepresent invention is controlled to be 0.0010˜0.0050%.

Ti can mainly fixate nitrogen. Ti can combine with N in the early stageof solidifying continuous casting billet, and forms TiN particles in thegrains, thereby to reduce the reactions between B and N, to fully exertthe effect of B on improving hardenability. The Ti content is controlledto be 0.020˜0.050%.

Al can mainly fixate nitrogen and deoxidize. The AlN formed by thecombination of Al and N can effectively refine grains. However,excessive addition can destroy the toughness of steel, and worsens thecasting performance of molten steel. The Al content of the presentinvention is controlled to be 0.015˜0.050%.

A further object of present invention is to provide the method ofmanufacturing the high-hardenability, medium-carbon, low-alloy roundsteel for fasteners. Special steps are as follows: the raw materials areprocessed, in sequence, by converter smelting or electric furnacesmelting, LF refining, RH/VD degassing to obtain molten steel, feedingTi wires and ferroboron after LF refining; the molten steel is castedinto a continuous casting billet by adopting low superheat degree of15˜40° C. and under a full argon protection, the continuous castingbillet is fed warmly at a temperature of 300˜600° C. or delivered to aslow-cooling pit and is slowly cooled for 32 hours or more, and then istaken out of the pit; the continuous casting billet is heated to1200˜1250° C. and kept warm for 3˜10 hours in the furnace, and then istaken out of the furnace; after high-pressure water descaling, thecontinuous casting billet is rolled in to round steel bar at atemperature of 1100˜1150° C.; the bar is processed by means of quenchingand tempering treatment in a continuous furnace thereby to obtainquenched and tempered round steel; the quenched and tempered round steelis able to be directly used in processing fasteners which meet ISO 898-1standard for grade 10.9 , such as bolts and the like.

Above quenching and tempering treatment includes quenching andtempering, the treatment can be performed in a roller hearth continuousfurnace or in induction lines, when in a roller hearth continuousfurnace, the quenching heating temperature is 840˜880° C., the time infurnace is 60˜360 min, the bar is water quenched by quenching ring; thetempering heating temperature is 530˜620° C., the time in furnace is300˜600 min, the bar is air-cooled outside the furnace to roomtemperature or is water-cooled to room temperature; when in inductionlines, the quenching heating temperature is 880˜950° C., the time infurnace is 3˜10 min, the bar is water quenched by quenching ring; thetempering heating temperature is 600˜700° C., the time in furnace is3˜10 min, the bar is air-cooled outside the furnace to room temperatureor is water-cooled to room temperature.

Compared with prior arts, the advantages of present invention focus on:taking a very small amount of alloy elements B and Ti to replace partsof noble metal elements Mo and Ni so as to improve hardenability ofsteel bar, manufacturing the fasteners with maximum diameter up to 65mm, and on the basis of satisfying the mechanical and physicalrequirements of quenched martensite content in steel core being morethan 90%, difference of the hardness of the whole cross-section beingwithin 30 HV, impact energy measured at −20° C. being z,28 27 J instandard ISO 898-1, obviously reducing the manufacture cost of alloy rawmaterial.

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

FIG. 1 is quenched and tempered core structure (100×) of embodiment 1;

FIG. 2 is quenched and tempered core structure (100×) of embodiment 2;

FIG. 3 is quenched and tempered core structure (100×) of embodiment 3.

MODE(S) FOR CARRYING OUT THE INVENTION

The present invention is further described in details with reference toembodiments.

Embodiment 1

The diameter of the steel for fastener relating to the presentembodiment is 64 mm, and the steel is processed with heat treatmentaccording to the requirements of 10.9 grade fastener, the chemicalconstituents by mass percentage are as follows: C: 0.42%, Si: 0.25%, Mn:0.95%, Cr: 1.10%, Mo: 0.10%, Ni: 0.12%, Cu: 0.12%, Al: 0.018%, B:0.0020%, Ti: 0.025%, the balance is Fe and unavoidable impurityelements.

The manufacturing processes are as follows: the main raw materials areprocessed, in sequence, by electric furnace smelting, LF refining, VDdegassing to obtain molten steel, feeding Ti wires and ferroboron afterLF refining; the molten steel is casted into a continuous casting billetby adopting low superheat degree of 15˜30° C. and under a full argonprotection, the continuous casting billet is slowly cooled for 32 hours;the continuous casting billet is heated to 1250° C. and kept warm for 4hours in the furnace, and then is taken out of the furnace; afterhigh-pressure water descaling, the continuous casting billet is rolledin to round steel bar at a temperature of 1150° C.; in a roller hearthcontinuous furnace, the bar is heated to 840° C. for 3 hours foraustenization, after austenization, the bar is high-press water quenchedby quenching ring, and the bar is further heated to 620° C. for 6 hoursfor tempering, the bar is air-cooled outside the furnace to roomtemperature. Comparison with the chemical constituents, the mechanicalperformance and the metallographic structure of 40CrNiMo steel which isusually used for manufacturing the fasteners with large size, is shownin table 1, table 2 and FIG. 1. It can be seen that Mo content and Nicontent in the present embodiment are greatly reduced, the noble metaladdition is less and at the same time the mechanical performance, thedistribution of hardness in transversal cross-section and themetallographic structure at the core all satisfy the requirements ofstandard ISO 898-1.

TABLE 1 Comparison of chemical constituents between CrB middle-carbonsteel in Embodiment 1 and round steel in Comparative Example 1 (wt %) CSi Mn Cr Mo Ni Cu Al Ti B Comparative 0.40 0.25 0.70 0.78 0.18 1.30 0.030.022 0.003 0.0001 Example 1 Embodiment 1 0.42 0.25 0.95 1.10 0.10 0.120.12 0.018 0.025 0.0020

TABLE 2 Comparison of performance after quenching and temperingtreatment between bar in Embodiment 1 and round steel in ComparativeExample 1 Hardness Yield Tensile of Vickers hardness of StrengthStrength Reduction −20° C. tempered transversal cross section Rp0.2 RmElongation of Area Akv core HV0.3 Steel Grade (MPa) (MPa) A % Z % J HRCSurface 1/2R Core Requirement

 930

 1040

 9

 48

 27

 30 + 1/2*C % 320-380 Comparative 1000 1100 13 52 62/56/60 55 354 348342 Example 1 Embodiment 1 970 1050 11 49 42/43/38 53 335 332 328 ※takea whole sample for measuring tensile properties; Vickers hardness oftransversal cross section: max-min ≤ 30HV.

Embodiment 2

The diameter of the steel for fastener relating to the presentembodiment is 54 mm, and the steel is processed with heat treatmentaccording to the requirements of 10.9 grade fastener, the chemicalconstituents by mass percentage are as follows: C: 0.37%, Si: 0.22%, Mn:0.95%, Cr: 1.10%, Mo: 0.08%, Ni: 0.10%, Cu: 0.10%, Al: 0.022%, B:0.0023%, Ti: 0.028%, the balance is Fe and unavoidable impurityelements.

Above round steel is processed, in sequence, by electric furnacesmelting, LF refining, VD degassing to obtain molten steel, feeding Tiwires and ferroboron after LF refining; the molten steel is casted intoa continuous casting billet by adopting low superheat degree of 15˜30°C. and under a full argon protection, the continuous casting billet isfed warmly at a temperature of 400° C.; the continuous casting billet isheated to 1200° C. and kept warm for 4 hours in the furnace, and then istaken out of the furnace; after high-pressure water descaling, thecontinuous casting billet is rolled in to round steel bar at atemperature of 1100° C.; in induction lines, the bar is heated to 880°C. for 5 minutes for austenization, after austenization, the bar isquenched by quenching ring, and the bar is further heated to 550° C. for5 hours for tempering in a roller hearth continuous furnace, the bar isair-cooled outside the furnace to room temperature. Comparison with thechemical constituents, the mechanical performance and the metallographicstructure of the conventional 40CrNiMo steel, is shown in table 3, table4 and FIG. 2. It can be seen that Mo content and Ni content in thepresent embodiment are greatly reduced, and at the same time themechanical performance, the distribution of hardness in transversalcross-section and the metallographic structure at the core all satisfythe requirements of standard ISO 898-1.

TABLE 3 Comparison of chemical constituents between CrB middle-carbonsteel in Embodiment 2 and round steel in Comparative Example 2 (wt %) CSi Mn Cr Mo Ni Cu Al Ti B Comparative 0.41 0.23 0.72 0.80 0.18 1.31 0.050.028 0.003 0.0001 Example 2 Embodiment 2 0.37 0.22 0.95 1.10 0.08 0.100.10 0.022 0.028 0.0023

TABLE 4 Comparison of performance after quenching and temperingtreatment between bar in Embodiment 2 and round steel in ComparativeExample 2 Hardness of Vickers hardness of Yield Tensile Reduction −20°C. tempered transversal cross Strength Strength Elongation of Area Akvcore section HV0.3 Steel Grade Rp0.2(MPa) Rm(MPa) A % Z % J HRC Surface1/2R Core Requirements

 930

 1040

 9

 48

 27

 30 + 1/2*C % 320-380 Comparative 1020 1120 14 55 65/60/68 55 350 348345 Example 2 Embodiment  980 1060 11 51 47/47/45 52 342 337 335 2 ※takea whole sample for measuring tensile properties; Vickers hardness oftransversal cross section: max-min ≤ 30HV.

Embodiment 3

The diameter of the steel for fastener relating to present embodiment is48 mm, and the steel is processed with heat treatment according to therequirements of 8.8 grade fastener, the chemical constituents by masspercentage are as follows: C: 0.36%, Si: 0.21%, Mn: 0.92%, Cr: 1.08%,Mo: 0.08%, Ni: 0.06%, Cu: 0.08%, Al: 0.023%, B: 0.0025%, Ti: 0.026%, thebalance is Fe and unavoidable impurity elements.

Above round steel is processed, in sequence, by KR pre-treatment,converter smelting, LF refining, RH degassing to obtain molten steel,feeding Ti wires and ferroboron after LF refining; the molten steel iscasted into a continuous casting billet by adopting low superheat degreeof 15˜30° C. and under a full argon protection, the continuous castingbillet is fed warmly at a temperature of 400° C.; the continuous castingbillet is heated to 1200° C. and kept warm for 4 hours in the furnace,and then is taken out of the furnace; after high-pressure waterdescaling, the continuous casting billet is rolled in to round steel barat a temperature of 1100° C.; in continuous induction lines, the bar isheated to 950° C. for 4 minutes for austenization, after austenization,the bar is quenched by quenching ring, and the bar is further heated to700° C. for 4 minutes for tempering, the bar is air-cooled outside thefurnace. Comparison with the chemical constituents, the mechanicalperformance and the metallographic structure of the conventional 4140MODsteel, is shown in table 5, table 6 and FIG. 3. It can be seen that Mocontent and Ni content in the present embodiment are greatly reduced,and at the same time the mechanical performance, the distribution ofhardness in transversal cross-section and the metallographic structureat the core all satisfy the requirements of standard ISO 898-1.

TABLE 5 Comparison of chemical constituents between CrB middle-carbonsteel in Embodiment 3 and 40CrNiMo round steel in Comparative Example 3(wt %) C Si Mn Cr Mo Ni Cu Al Ti B Comparative 0.41 0.25 0.95 1.05 0.231.31 0.05 0.028 0.003 0.0001 Example 3 Embodiment 3 0.36 0.21 0.92 1.080.08 0.06 0.08 0.023 0.026 0.0025

TABLE 6 Comparison of performance after quenching and temperingtreatment between Embodiment 3 and 40CrNiMo round steel Yield TensileHardness of Vickers hardness of Strength Strength Reduction −20° C.tempered transversal cross Rp0.2 Rm Elongation of Area Akv core sectionHV0.3 Steel Grade (MPa) (MPa) A % Z % J HRC Surface 1/2R CoreRequirements

 660

 830

 12

 52

 27

 30 + 1/2*C % 255-335 Comparative 790 900 18 60 100/98/90 56 288 283 279Example 3 Embodiment 3 760 880 15 56  78/72/65 52 280 280 270 ※take awhole sample for measuring tensile properties; Vickers hardness oftransversal cross section: max-min ≤ 30HV.

Besides above embodiments, the present invention further includes otherembodiments, and any technical solution formed by equivalenttransformation or equivalent substitution shall fall within theprotection scope of claims of the present invention.

1. A high-hardenability, medium-carbon, low-alloy round steel forfasteners, wherein its chemical constituents by mass percentage are asfollows: C: 0.36˜0.44%, Si: 0.15˜0.40%, Mn: 0.80˜1.00%, Cr: 1.00˜1.15%,Mo: 0.05˜0.25%, Ni: 0.05˜0.25%, Cu: 0.05˜0.25%, Al: 0.015˜0.050%, B:0.0010˜0.0050%, Ti: 0.020˜0.050%, the balance is Fe and unavoidableimpurity elements; the maximum diameter of the round steel is 65 mm,after quenching and tempering treatment, its mechanical structure andperformance meet the requirements of ISO 898-1 standard or otherequivalent standards for grade 10.9 fasteners, specific requirements areas follows: quenched martensite content is z,27 90%, after quenching andtempering treatment, when a tensile sample is tested, the diameter of atensioned part of the sample is 75% of nominal size, yield strength ofthe round steel Rp0.2 is z,27 3940 MPa, tensile strength Rm is 1040˜1140MPa, elongation is z,27 9% , reduction of area at fracture is z,27 48%,Charpy impact energy measured at −20° C. is

27 J when a Akv2 impact sample is taken at ½ radius, HV0.3 hardness ofthe whole cross-section of round steel is 320˜380 HV, difference of thehardness of the whole cross-section is within 30 HV.
 2. A method ofmanufacturing the high-hardenability, medium-carbon, low-alloy roundsteel for fasteners according to claim 1, wherein the raw materials areprocessed, in sequence, by converter smelting or electric furnacesmelting, LF refining, RH/VD degassing to obtain molten steel, feedingTi wires and ferroboron after LF refining; the molten steel is castedinto a continuous casting billet by adopting low superheat degree of15˜40° C. and under a full argon protection, the continuous castingbillet is fed warmly at a temperature of 300˜600° C. or delivered to aslow-cooling pit and is slowly cooled for 32 hours or more, and then istaken out of the pit; the continuous casting billet is heated to1200˜1250° C. and kept warm for 3˜10 hours in the furnace, and then istaken out of the furnace; after high-pressure water descaling, thecontinuous casting billet is rolled in to round steel bar at atemperature of 1100˜1150° C.; the bar is processed by means of quenchingand tempering treatment in a continuous furnace thereby to obtainquenched and tempered round steel; the quenched and tempered round steelis able to be directly used in processing fasteners which meet ISO 898-1standard for grade 10.9, such as bolts and the like; the above quenchingand tempering treatment includes quenching and tempering, the treatmentcan be performed in a roller hearth continuous furnace or in inductionlines, when in a roller hearth continuous furnace, the quenching heatingtemperature is 840˜880° C., the time in furnace is 60˜360 min, the baris water quenched by quenching ring; the tempering heating temperatureis 530˜620° C., the time in furnace is 300˜600 min, the bar isair-cooled outside the furnace to room temperature or is water-cooled toroom temperature; when in induction lines, the quenching heatingtemperature is 880˜950° C., the time in furnace is 3˜10 min, the bar iswater quenched by quenching ring; the tempering heating temperature is600˜700° C., the time in furnace is 3˜10 min, the bar is air-cooledoutside the furnace to room temperature or is water-cooled to roomtemperature.